Storage media capable of high-density storage such as optical disks are recently in great use. They are required to have high flatness in order to achieve a higher storage density. For this purpose, the surface shape of a storage medium must be checked in a manufacturing process. As a method of performing such surface shape measurement, a moire method has been known. The moire method is a method of measuring the surface shape of an object from the moire fringes (contour lines of a surface shape) produced by superimposing a grating and a grating image deforming in accordance with the shape of the object when light from a point light source passes through the grating and strikes the object.
In the moire method using divergent light, however, the contour line interval (a level difference per moire fringe) increases with an increase in the distance between a grating and an object. This causes an error unless the ordinal number (degree n) of a given moire fringe from the grating surface can be specified. In addition, in the moire method, since only contour lines are displayed, recesses and projections cannot be discriminated. The measurement precision can be improved by reducing the pitch of the grating. If, however, the pitch is reduced, the contrast of the moire fringes decreases. This limits the contour line interval to about 10 μm at most.
In order to solve such a problem, therefore, a scheme based on a combination of a parallel light moire method and a phase shift method has been proposed (e.g., Japanese Patent Laid-Open No. 7-332956). A characteristic feature of the parallel light moire method is that light from a point light source 21 is converted into parallel light by using a lens 22 to always make the contour line interval constant regardless of the distance from a grating 23, as shown in FIG. 8. For this reason, there is no need to determine a degree n of a moire fringe, and no error based on the contour line interval is caused. When reflected light is used, a contour line interval Δh is obtained by only an incident (exit) angle θ of light and a pitch p of the grating 23 according to the following equation:Δh=p/(2 tan θ)  (1)Referring to FIG. 8, reference numeral 24 denotes a condensing lens for condensing reflected light.
In the conventional moire method using divergent light, an object having a mirror-reflecting surface such as a glass member or silicon wafer cannot be measured because the reflection angle changes in accordance with the incident angle which changes depending on the position (an object having a diffused reflecting surface can be measured because the angle seen by an observer becomes a reflection angle). In contrast to this, according to the parallel light moire method, since an incident angle and reflection angle remain the same regardless of the position, even a mirror surface object can be measured.
In the phase shift method, assuming that discrete information such as contour line fringes is a periodic trigonometric function of a light intensity, the information is handled as continuous information, i.e., the phase of the trigonometric function, to recognize a surface shape with a precision higher than the number of contour line fringes. The phase shift method is disclosed in, for example, Tomizawa and Yoshizawa, “Phase-Shifting Shadow Moire Method”, Proceedings of JSPE Fall Meeting (1991), p. 677.